Millimeter wave generator



July 15, 1958 l/ j/ z i 2 M5 ////5 ,/,,yi, ,f

MIME/VTOKS E0 wA/m A7. JUH/VSUA/mva KARL 5. HEH/VflV/ST MILLIMETER WAVE GENERATOR Edward 0. Johnson and Karl J. Hernqvist, Princeton,

N. 1., assignors to Radio Corporation of America, a corporation of Delaware Application October 21, 1955, Serial No. 541,946

9 Claims. (Cl. 250-17) This invention relates to wave generators. In particular this invention relates to wave generators that generate super. high frequencies of approximately one millimeter wave length.

In the past there have been many attempts at producing a millimeter wave generator. Certain types of gas discharge devices have been tried for this purpose. It has been found that the plasma density in those gas discharge devices that have been used was far too low to generate waves in the millimeter wave range. Another type of device that has been considered for a millimeter wave generator is the solid state, semi-conductor type of device. Solid state theory has shown that the collision frequency of the particles in this type of device is too high, except at extremely low temperatures, to produce millimeter waves.

It is therefore an object of this invention to provide a new and improved millimeter wave generator.

It is a further object of this invention to provide a novel wave generator that is capable of producing waves at frequencies of the order of 300 kilo-megacycles.

These and other objects are accomplished in accordance with this invention by projecting an electron stream into a region of extremely dense plasma within the envelope of a gas discharge device. In the embodiment to be illustrated the region of extremely dense plasma is constituted by the cathode spot on the surface of the liquid mercury in a mercury vapor discharge device. The velocity of the electron stream and the size of the dense region are correlated so that the region functions similarly to a resonant cavity, and the stream of electrons produces plasma oscillations within the region that are of a wavelengthin the millimeter range.

The invention will be more clearly understood from reading the following specification in conjunction with the accompanying single sheet of drawings wherein:

Figure l is a sectional view of a wave generator in accordance with this invention; and,

Figure 2 is a top view taken along line 22 of Figure 1.

Referring now to the drawings in detail, there is shown a discharge device 10 which comprises an evacuated enclosure 12 having'a re-entrant stem portion 14 forming a part of the enclosure 12. Also forming a part of the enclosure 12 is a window 16, which is permeable to waves in the millimeter range, such as quartz. The window 16 may be sealed to the enclosure by means of a graded seal 17 as is well known. As can be seen from Figure l the enclosure 12 is divided into two compartments by means of an apertured plate, or bafile, 18; with the two compartments communicating one with the other through a small aperture 20 in plate 18.

'In one compartment, i. e. the upper compartment in Figure 1, of the device 10 there is provided an electron gun structure 22, that includes a cathode 24, control electrode 26 and a final accelerating electrode 28, for forming a high velocity electron beam 30. The beam 30 United States Patent 2,843,732 Patented July 15, 1958 5.4 is directed from the gun 22 through aperture 20 into the other portion of the enclosure 12.

Within the other portion, i. e. the lower compartment as shown in Figure 1, of the enclosure 12, there is provided a low boiling point metal 32 that may take the form of a material such as gallium or mercury. Extending through the low boiling point metal, hereinafter referred to as mercury pool, is a wetted anchoring electrode 34 which has a pointed end that extends slightly above the surface of the mercury pool and is surrounded by an anode 36. The anode 36 is a U-shaped to function as a radiation reflector to direct radiations through the quartz window 16.

The pressure within the device 10 varies depending upon which compartment is being considered. The pressure adjacent the mercury pool 32 may be, for example, approximately microns of mercury, while the pressure adjacent the gun 22 may be, for example, approximately l0 millimeters of mercury. The pressure in the cathode spot, or ball-of-fire discharge, which will be explained hereinafter, may be as much as a few millimeters of mercury. In order to maintain this pressure differential a coolant 38, such as liquid air or liquid nitrogen, is circulated around the gun 22 in the re-entrant portion 14 of enclosure 12. Due to this coolant, any mercury vapor within the electron gun compartment is condensed, and the vapor pressure within this portion of the enclosure is much lower than that within the other portion of the enclosure, so that high voltages may be used on the electron gun 22 to produce a high velocity electron beam 30.

The elements of gun 22 may be formed by any conventional electron gun that is capable of producing a high velocity electron beam 36. The beam 30 may be a direct current beam, or may be modulated by signal information being applied to the control electrode 26. The cathode 24 may be coated with any known electron emissive material such as barium or strontium carbonate. The low boiling point metal 32 may be pure mercury, or'may include various impurities as is well known. Also, rare gases may be used in the envelope such as zenon to vary the gas pressure in the lower compartment. The anchoring electrode 34, which extends approximately 1 millimeter above the mercury pool 32, may be made of a material such as molybdenum or nickel. The spacing between the mercury surface and the anode 36 may be approximately two millimeters. The spacing between the final accelerating electrode 28, and the apertured plate 18, may be approximately two centimeters. The aperture 24) in plate 18 may be approximately 30 mils in diam eter. The spacing between anode 36 and the plate 18 may be approximately two millimeters. The quartz window 16 may be sealed to the envelope 12 by any known type of seal such as the graded seal 17.

During operation of the device 10, a direct current potential diiference such as that shown in Figure 1, and represented by battery 37, is applied between the mercury cathode 32 and the anode 36 to cause ionization of the mercury within the lower compartment of device 10. The discharge in a mercury vapor pool type of device occurs from a very small region, or cathode spot, 39 at the surface of the mercury pool. This discharge is very similar to a type of gas discharge known as the ball-offire mode of discharge. This mode of discharge is de* scribed in an article in the RCA Review of September 1951, entitled Studies of externally heated hot cathode arcs, by L. Malter, E. 0. Johnson and W. M. Webster.

Briefly, in a discharge of this type, the ball, or cathode spot 39, is made up of an extremely dense plasma of a substantially equal number of free electrons and free positive ions. In regions of the tube removed from the ball or cathode spot, but still in the lower compartment,

a less dense plasma is formed. Thermal energy of the particles within the ball prevent recombination of the free ions and free electrons, which are moving at high velocity within the ball. When the electron beam 3% is projected into the ball, the particles Within the ball tend to bunch, or density modulate the electron beam ina manner similar to that occurring in a velocity modulation device or klystron. In this analogy, the resonant cavity structure of the klystron is the plasma itself, and

the resonant frequency is determined by the density of the plasma. The resonant action within the call is believed to be caused by the combined action of the inertial mass of the particles Within the ball or cathode spot and the beam 30, that causes displacement of the particles from their average spacing so as to cause oscillation.

The size and density of the ball of fire within a mercury pool discharge device may be controlled to some extent by the current drawn between the mercury pool and the anode. Also, the size of the ball may be controlled to some extent by the vapor and gas pressure. parameter is difficult to control in a mercury pool device since the'pressure in the ball is much higher than in other portions of the tube, i. e. near the anode 36. However, the pressure may be influenced by controlling the temperature of the discharge section of the tube and also by utilizing a combination of a gas and a vapor.

The positive ions-produced within the ball serve to provide a plasma throughout the lower section of the tube. The plasma serves as a low impedance conductor permitting large electron currents to flow from the cathode 32 to the anode 36. The ball-of-fire mode is explained in detail in the above identified article.

*In the device 10 the ball-of-fire is anchored to the anchoring electrode 34 by the wetting of the anchoring electrode 34. This Wetting action occurs because the metal surface attracts mercury atoms more strongly than the mercury surface attracts its own atoms. The wetting action occurs when the anchoring electrode 34 has been thoroughly cleaned. The anchoring effect is believed to occur primarily because the surface of the mercury is stable on the wetted area, which is not the case on the free surface of the pool.

Since the ball is the region where most of the ionization occurs, the plasma density in this region is extremely large. Substantially, all of the current conducted through the vapor part of the tube passes through the ball or cathode spot. Therefore, the plasma density Within the ball may be determined by the following equation:

as l e A where: N is the plasma density in electrons per cubic centimeter; e is the charge on an electron in coulombs; I is the total current drawn through the limiting resistor 35 in amperes; A is the cross-sectional area of the ball in square centimeters; and v is the average velocity of the electrons traveling within the ball, which for a given tube is substantially constant and is approximately 6 10 centimeters per second with an energy of one volt.

Since plasma oscillations within a ball occur at a frequency that depends upon the density of the plasma, the frequency of the energy produced may be determined by the following relation:

where f is the frequency of oscillation in cycles, and N is the plasma density in electrons per cubic centimeter.

As can be seen from the above relation, in order to obtain radiations of l millimeter wavelength, corresponding to a frequency of 300 kilo-megacycles, N must be approximately 10 particles per cubic centimeter. Plasma densities of this order have been obtained in such mercury pool discharge devices.

The electron beam 3d is directed toward the anchoring electrode 34 to which the ball 39 is anchored. Due

This latter to the resonance of the ball, the electron beam is density modulated by the ball plasma and the modulated beam reacts back on the ball plasma to generate radiations of extremely high frequencies. The higher the density of the particles the higher the frequency of radiation. Thus, the frequency of radiation from the ball may be controlled by controlling the plasma density within the ball. The plasma density within the ball may be controlled to some extent by the electron current to the anode 36, as was explained.

The radiations from the ball are directed by means of the anode 36 through the quartz window 16. It should be understood that anode 36 is shown as a U-shaped member merely to illustrate that a radiation focusing device may be used to focus the radiations developed by the beam 16 striking the ball-of-fire discharge. The output radiations are of such a high frequency that they would be utilized in optical systems such as quartz reflectors, metal reflectors, or the like.

The term millimeter range is intended to refer to Wavelength of the order of 1 millimeter, and may include wavelengths from a fraction of a millimeter to several millimeters.

What is claimed is:

1. A millimeter wave generating device comprising means for producing a dense plasma, said plasma having a density of the order of 10 particles per cubic centimeter, and means for directing an electron stream into said plasma.

2. A wave generating gas discharge device comprising a loW melting point metallic cathode, means for producing an ionizing discharge'from said cathode whereby a plasma is formed, means for anchoring the cathode spot of said discharge from said cathode, and means for directing a stream of electrons into said cathode spot.

3. A millimeter Wave generator comprising asealed enclosure containing a low melting point metal cathode, an anode spaced from said cathode, terminal means for applying a potential difference between said cathode and said anode for producing a discharge from said cathode to said anode, means for anchoring said discharge to a restricted region on the surface of said cathode, and electron gun means for directing an electron beam toward said region.

4. A millimeter wave generator device comprising a sealed enclosure containing a vapor pool, an anode spaced from said pool, terminal means for applying a potential difference between said pool and said anode to produce a vapor discharge, electrode means for anchoring said discharge to a restricted position on the surface of said pool, an electron gun for producing an electron beam, means for directing said beam into said position, and means for directing radiations formed by the interaction of said beam and said discharge through a portion of said enclosure.

5. A millimeter wave generating device comprising a low melting point metallic cathode, .an anode, means for applying a potential difference between said cathode and said anode for producing a discharge of the type including a high plasma density cathode spot, and electron gun means for directing the stream of electrons at said cathode spot.

6. A millimeter wave generating device comprising a sealed enclosure, a material permeable to energy of millimeter wave lengths forming a portion of said enclosure, a mercury pool within said enclosure, an anode spaced from said pool and within said enclosure, terminal means for applying a potential difference between said pool and said anode for producing a discharge of the type including a cathode spot between said pool and said anode, means for anchoring the cathode spot of said discharge, and means for producing an electron beam and directing said beam into said cathode spot whereby radiations are produced, the frequency of said radiations verying with the plasma density of said Cathode spot.

7. A millimeter wave generating device comprising a sealed enclosure, a material that is permeable to energy of millimeter wave lengths forming a portion of said enclosure, an electron gun for producing an electron beam within said enclosure, means for cooling said enclosure adjacent to said electron gun, a mercury pool in said enclosure, an anode adjacent to said pool and in said enclosure, means for producing a vapor discharge from said pool to said anode, means for anchoring the cathode spot of high plasma density on said pool, and means for directing said electron beam into said high density spot.

8. A millimeter wave generating device comprising a sealed enclosure, a material permeable to radiations of millimeter wave length range forming a portion of said enclosure, said enclosure being divided into sections by an apertured plate, a mercury pool within one section of said enclosure, terminal means for applying a potential difference between said pool and said anode for producing a discharge between said pool and said anode, metallic means for anchoring the cathode spot of said discharge, and means within another section of said enclosure for producing an electron beam and directing said beam through an aperture in said plate and at said metallic means.

9. A millimeter wave generating device comprising a sealed enclosure, a quartz window forming a portion of said enclosure, an apertured metallic plate dividing said enclosure into two sections, said plate having a single aperture therein, a mercury pool in one of said sections, a U-s'haped anode spaced from said pool and in said one of said sections, terminal means for applying a diiference in potential between said pool and said anode for producing an ionizing discharge therebetween, pointed metallic means extending'through said pool for anchoring the cathode spot in said discharge, an electron gun within another section of said enclosure for producing an electron beam, means for directing said beam through said aperture and at said cathode spot whereby radiations are generated, and said U-shaped anode being adjacent to said window whereby said radiations" are reflected and focused by said anode through said window.

References Cited in the file of this patent UNITED STATES PATENTS 2,201,003 Berkey May 14, 1940 2,459,199 Stutsman Jan. 18, 1949 2,643,297 Goldstein et al June 23, 1953 

